Melt 1/2 cup butter into a heavy skillet over low heat. Blend in 1 cup brown sugar and 1/4 cup cream, stirring constantly. Bring to a boil and cook until completely dissolved, about 1 minute. Remove from heat and transfer to a bowl. Cool to lukewarm, 110 degrees. Gradually add 1 1/2 cups sifted powdered sugar (may need more). Beat until thick enough to spread.